There have been various proposals to use Asynchronous Transfer Mode (ATM) as the transfer method to provide a variety of services to end users by satellite or spacecraft networks. ATM is a standardized service, which defines a number of service categories. The service category which is provided to each customer depends upon the quality-of-service agreement with the customer. One of the categories of service is the Available Bit Rate (ABR) service, which provides access to network or transmission path bandwidth which is not used for other services. Naturally, within such a class of service, the available bandwidth may vary substantially from time to time. A measure of quality in ATM service is the number or percentage of data cells which are lost or damaged during transmission. It will be understood that, if the source of ATM signals produces signals having a bandwidth greater than that which can be accommodated within the ABR bandwidth at the time of transmission of the data, some data will be lost or distorted, which is the equivalent of lost. In order to prevent overloading of the ABR-available bandwidth, a feedback scheme is used in ATM ABR service, in which Resource Management cells are interspersed with the data cells at the ATM signal source. According to ATM standards, these Resource management cells follow every 32 data cells. These Resource Management cells have bit locations for aiding in control of the ABR transmission rate, so as to accommodate the currently available bandwidth of the ABR service. At various locations along the ATM transmission path, the various switches through which the data flows can mark the bit locations of the Resource Management cells to indicate the existence of congestion. The Resource Management cells have locations for explicit forward congestion indication (EFCI) marking, for relative rate marking, and for explicit rate marking.
The Explicit Forward Congestion Indication marking bit locations associated with the Resource Management cells are intended to provide backward compatibility with one type of congestion control mechanism in which a switch through which the ATM data passes is capable of setting the state of a header bit which represents, by a first state, the lack of congestion, and the second state represents the existence of congestion. Once set to the “congestion” state, downstream switches do not restore the “no congestion” state. Such information, when fed back to the source of ATM data by a stream of return or back Resource Management (BRM) cells, can be used to repeatedly decrease the data rate, until such time as the back Resource Management cells no longer indicate the existence of congestion. The response of this type of feedback control depends, at least in part, on the length of the transmission path, and the delay occurring before return of the Resource Management cells carrying the congestion information. The Relative Rate marking bit locations associated with Resource Management cells are intended to provide backward compatibility with another type of congestion control mechanism, in which the switch through which the ATM data passes sets a congestion indication (CI) bit and a bit representing an increase or no increase (NI) of the Allowable Cell Rate (ACR). The CI bit is equivalent to the EFCI bit in this convention, and the source of data should decrease its bit rate when the CI bit is set. The NI bit, when set, indicates to the source of data that it should not increase its Available Cell Rate. The explicit rate marking bit of a Resource Management cell specifies the actual ACR that the source should use, although it should be noted that each switch of the system may decrease the indicated rate, depending upon the conditions which it experiences.
While Asynchronous Transfer Mode protocols provide for bandwidth control by setting of one of the various EFCI, CI, and Relative rate bits of the Resource Management cells, existing spacecraft have simple switches which are incapable of setting bandwidth control bits. Because of cost considerations, even new spacecraft may not be equipped with switches which have bandwidth control bit setting capability.
It would be desirable to be able to extend ATM ABR service to spacecraft with simple switches.